1. Field of the Invention:
The present invention relates to a cover-lay film for high-density mounting which exhibits satisfactory performance in adhesion at relatively low temperatures, peel strength, solder heat resistance, moisture resistance, circuit space filling capabilities, and electrical reliability and to a printed circuit board obtained by using the cover-lay film.
2. Description of the Related Art:
With the recent drastic development in the field of electronics, size and weight reduction and density increase of electronic equipment especially for communications systems and consumer products have been observed, and the performance requirements have become more exacting. In this connection, flexible printed circuit boards (hereinafter xe2x80x9cPCBsxe2x80x9d), featuring flexibility and repeated bending endurance, enable three-dimensional and high-density mounting in a limited space and have been broadening their applications to wiring of electronic equipment, cables and composite components having a connector function. Flexible PCBs are fabricated of a flexible base material on which a circuit pattern is formed in a usual manner. The circuit is protected with a cover-lay film for some applications.
A base material for flexible PCBs is an integral laminate composed of an insulating base film having high heat resistance and excellent electrical and mechanical characteristics and a metal foil bonded with an adhesive. Characteristics required of the base material for flexible PCBs include adhesion, intimate contact capabilities, heat resistance, electrical properties, and processability.
The latest tendencies, such that chips-on-flex (COFs, IC chips directly attached onto a flexible printed wiring board) have been put to practical use and that base materials for flexible PCBs have come to be adopted as an interposer of chip-scale-packages (CSPs) and multichip modules (MCMs), have demanded further improvements on heat resistance and moisture resistance of base materials for flexible PCBs as a semiconductor package constituent.
A cover-lay film is provided for circuit protection, improved flexibility of flexible PCBs, and the like. Cover-lay films include film base types composed of an insulating base film having an adhesive applied to one side thereof, dry film types of which the adhesive layer also serves as an insulation layer, and liquid types. These cover-lay films are required to have storage stability, intimate contact capabilities, heat resistance, electrical properties, and processability.
Available cover-lay films having an epoxy resin composition as an adhesive have low electrical reliability while satisfactory in adhesion, moisture resistance, circuit space filling capabilities, and solder heat resistance. This drawback is, as is pointed out, attributed to inorganic ions used in the synthesis of a butadiene-acrylonitrile copolymer which is usually compounded into an epoxy resin.
A cover-lay film having a pyromellitic acid-based polyimide film (e.g., Kapton H from E. I. du Pont) as a base film coated on one side with an epoxy resin adhesive is known. However, such a base film has low dimensional stability, and the mechanical properties of the cover-lay film greatly reduce on moisture absorption, which make the cover-lay film unfit for high-density mounting.
An object of the present invention is to provide a cover-lay film for high-density mounting which is capable of sticking at relatively low temperature to exhibit satisfactory adhesion, satisfactory in solder heat resistance, moisture resistance, and circuit space filling capabilities, and excellent in electrical characteristics, dimensional stability, and mechanical properties. Another object is to provide a circuit board having the cover-lay film.
The present invention provides a cover-lay film comprising a heat-resistant film and an adhesive layer, wherein the adhesive layer comprises an epoxy resin composition containing (a) an epoxy resin, (b) a curing agent, (c) a phenolic hydroxyl-containing polyamide-poly(butadiene-acrylonitrile) copolymer, and (d) an ion capturing agent and having a glass transition temperature of 80xc2x0 C. or higher after cure, the heat-resistant film is a polyimide film mainly comprising a polyimide containing a 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid component and a p-phenylenediamine component and having a coefficient of linear thermal expansion of 10 to 25 ppm/xc2x0 C. in a temperature range of 50 to 200xc2x0 C. in both machine and transverse directions and a tensile modulus of 450 to 1100 kgf/mm2 in both machine and transverse directions, and the cover-lay film has such electrical reliability that the surface insulation resistance measured at 121xc2x0 C. and 85% RH with an applied voltage of 50 V using a comb pattern of IPC-B-25 from level B up, preferebly from level A up, shows no reduction for more than 200 hours.
The present invention also provides a circuit board produced by using the cover-lay film.